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Mercedes Rules Out Three-Cylinder Engines for its RWD Models

Mercedes-Benz says it won’t use three-cylinder petrol and diesel engines for its core range of rear-wheel-drive sedans, although rival BMW is adopting the technology on the i8 plug-in hybrid sports car.

“We have looked at three-cylinder engines, but there are too many compromises, such as refinement and the savings aren’t that significant,” Bernhard Heil, head of engine development, was quoted as saying by Autocar at the Geneva Motor Show.

The engineer says Mercedes-Benz has tested three-cylinder engines but has ruled them out because of the vibration and loss of refinement, particularly at low speed operation when fuel savings could be maximized. The carmaker is tuning its engines to operate at as low speed as practical, a strategy called “Down-Speeding” by Heil.

To solve the refinement problem, 3-cylinder engines need balancer shafts, but these add weight and cost, cancelling many of the advantages of small displacement engines.

Heil admitted however that Mercedes is likely to develop a second family of slightly smaller-capacity diesels with a lower displacement that today’s 2.1-liter OM651 unit. He didn’t mention a displacement, but 1.9- or 2.0-liter units are possible.

As for its FWD lineup of cars, Mercedes-Benz could offer three-cylinder engines in the future, albeit with balancer shafts. According to Heil, the packaging advantages would also allow Mercedes to place a motor/generator unit between the front wheels for future hybrid versions.